Optimization of a secondary VOI protocol for lung imaging in a clinical CT scanner

Abstract

We present a solution to meet an unmet clinical need of an in‐situ “close look” at a pulmonary nodule or at the margins of a pulmonary cyst revealed by a primary (screening) chest CT while the patient is still in the scanner. We first evaluated options available on current whole‐body CT scanners for high resolution screening scans, including ROI reconstruction of the primary scan data and HRCT, but found them to have insufficient SNR in lung tissue or discontinuous slice coverage. Within the capabilities of current clinical CT systems, we opted for the solution of a secondary, volume‐of‐interest (VOI) protocol where the radiation dose is focused into a short‐beam axial scan at the z position of interest, combined with a small‐FOV reconstruction at the xy position of interest. The objective of this work was to design a VOI protocol that is optimized for targeted lung imaging in a clinical whole‐body CT system. Using a chest phantom containing a lung‐mimicking foam insert with a simulated cyst, we identified the appropriate scan mode and optimized both the scan and recon parameters. The VOI protocol yielded 3.2 times the texture amplitude‐to‐noise ratio in the lung‐mimicking foam when compared to the standard chest CT, and 8.4 times the texture difference between the lung mimicking and reference foams. It improved details of the wall of the simulated cyst and better resolution in a line‐pair insert. The Effective Dose of the secondary VOI protocol was 42% on average and up to 100% in the worst‐case scenario of VOI positioning relative to the standard chest CT. The optimized protocol will be used to obtain detailed CT textures of pulmonary lesions, which are biomarkers for the type and stage of lung diseases.

Clinical CT underestimation of the percentage volume occupied by cysts in patients with lymphangioleiomyomatosis

“…In this study, we found that the standard high-resolution helical volume chest CT scan with standard reconstruction resolution underestimated the cyst score, due to inability to resolve small cysts, which became visible in high resolution images from a UH re-scan. The UH rescan was optimized in a previous study on a chest phantom that contained lung mimicking and control foams [17]. That study concluded that the UH re-scan maintained the same noise level as the standard HRCT images but at twice the resolution.…”

“…With regard to the question whether the difference between the standard HRCT scan and the UH re-scan is simply due to the resolution of image reconstruction, this was assessed quantitatively in the previous chest phantom study [17], which found that the HRCT scan reconstructed at the resolution of the UH re-scan had 2.4 times the noise level of the UH re-scan. The increase in noise was consistent with the relative levels of dose concentration between the two scans.…”

“…The HRCT helical volume scan used a tube setting of 120 kV/R700 mA and a rotation speed of 0.275 s giving a total scan time of 3.6 s to cover 360 mm with a helical pitch of 0.813. The short zlength UH re-scan was an axial scan (static patient bed) with a tube setting of 120 kV/350 mA, two rotations at a speed of 1.5 s giving a total scan time of 3 s, and covering 20 mm [17].…”

“…The focus of this study is on the accuracy of the cyst score by standard high-resolution helical volume chest CT, using as reference an ultra-high resolution short z-length re-scan that provides extra tissue resolution [17]. The accuracy of cyst scoring depends on image resolution to differentiate between cyst, parenchyma, and image noise.…”

“…Limited or reduced Z length scans have been used previously in abdominal, neck and chest CTs in both adults and children as a way to reduce radiation dose T [18][19][20][21][22][23][24]. The specific UH re-scan of this study has been optimized for best resolution in a systematic chest phantom study previously [17]. Typically, the UH re-scan deposits 40% the dose of a chest HRCT scan into a short z-length of 2 cm in axial scan mode, resulting in doubling the resolution at the same contrast-to-noise ratio.…”